The present invention relates to a DC-DC converter suitable for a power supply circuit, specifically to a DC-DC converter with an improved efficiency.
Video equipments in recent years such as a camcorder, a digital still camera (DSC) and a mobile phone with DSC use charge-coupled devices (CCDs) to capture an image. A power supply circuit that provides both positive and negative high voltages (over 10 volts) and high current (several milliamperes) is required in order to drive the CCDs. A switching regulator is used for that purpose today.
The switching regulator can generates the high voltages with high performance, i.e. with high power efficiency. However it has a drawback to generates a harmonic noise when switching a current, and therefore the power supply has to be used with a noise shield. Another drawback with the switching regulator is a difficulty in reducing the size of the equipment, since it requires a coil as an external part.
Switched capacitor type DC-DC converters have been proposed in order to overcome the drawbacks mentioned above. An example of the DC-DC converters of the kind is reported in the Journal of Institute of Electronics, Information and Communication Engineers, (C-2 Vol.J81-C-2 No.7 pp.600-612, July 1998).
FIG. 9 and FIG. 10 are circuit diagrams of a switched capacitor type DC-DC converter of the prior art. A voltage source 10 provides a supply voltage Vdd. Each of the capacitors C1, C2 and C3 composes each stage of the DC-DC Converter. Each of switches 11, 12 and 13 is connected between the power supply Vdd and one end of each of the capacitors respectively, and each of switches 21, 22 and 23 is connected between a ground (0V) and the other end of each of the capacitors respectively.
A switch 30 is disposed between the power supply Vdd and a ground (0V) side end of the capacitor C1. A switch 31 is disposed between a Vdd side end of the capacitor C1 and a ground (0V) side end of the second stage capacitor C2. A switch 32 is disposed between a Vdd side end of the capacitor C2 and a ground (0V) side end of the third stage capacitor C3. A switch 33 is disposed between a Vdd side end of the capacitor C3 and an output terminal 40. Cout is an output capacitor. A current load 50 is connected to the output terminal 40. Operation of this three-stage DC-DC converter will be described hereafter.
The switches 11-13 and 21-23 are turned ON, and the switches 30-33 are turned OFF, as shown in FIG. 9. The capacitors C1-C3 are connected in parallel between the power supply Vdd and the ground (0V). Each of voltages V1-V3 of each of the respective capacitors C1-C3 is charged to Vdd. Given that an output current from the output terminal 40 is Iout, a charging current to each of the capacitors is 2 Iout.
Next, as shown in FIG. 10, the switches 11-13 and the switches 21-23 are turned OFF, and the switches 30-33 are turned ON. Then the capacitors C1-C3 are connected in series with each other while they are disconnected from the power supply Vdd and the ground (0V), and discharging takes place. The voltage V1 is boosted to 2 Vdd, the voltage V2 is boosted to 3 Vdd and the voltage V3 (=Vout) is boosted to 4 Vdd, due to a capacity coupling effect. Given that the output current from the output terminal 40 is Iout, a current from the power supply Vdd to the capacitor C1 is 2 Iout.
As described above, the switched capacitor type DC-DC converter generates as high voltage as 4 Vdd with the output terminal 40 when provided with the power supply voltage of Vdd.
A theoretical efficiency xcex7 of a DC-DC converter is defined as output power/input power. Assuming that duration of a status of FIG. 9 and duration of a status of FIG. 10 are equal, and neglecting all voltage loss due to the switches and other factors,
Input power=4xc3x972 Iout/2xc3x97Vdd=Ioutxc3x974 Vdd
Output power=Ioutxc3x974 Vdd
Therefore the theoretical efficiency xcex7 is 100%.
In general, n-stage switched capacitor type DC-DC converter provides an output voltage of (n+1) Vdd.
However the conventional switched capacitor type DC-DC converter provides a boosted voltage in increments of Vdd only. When the switched capacitor type DC-DC converter is used as a power supply circuit, a step-down voltage adjustment is made by a regulator in order to adjust the output voltage to a desired voltage. A drawback of this method is a decline in the efficiency of the power supply circuit, especially when the discrepancy between the desired voltage and the output voltage of the DC-DC converter of (n+1) Vdd is large.
An objective of this invention is to improve the efficiency of a power supply circuit by providing a DC-DC converter capable of generating a boosted voltage in increments of less than a supply voltage of Vdd, for instance, 1.5 Vdd, 2.5 Vdd or 3.5 Vdd.
The DC-DC converter of this invention comprises a plurality of capacitors in at least one stage and switches to alternate the connection of the capacitors in series and in parallel. The switches connect a plurality of capacitors in series when charging them and connect them in parallel when discharging them.
Since the second capacitors are connected in series during charging, each of the second capacitors is charged to a divided voltage (0.5 Vdd when two capacitors are disposed, for instance). And then the divided voltage is transferred to the next stage by a capacity coupling, as the second capacitors are connected in parallel during discharging. By doing so, it is made possible to generate an output voltage in increments of less than the supply voltage Vdd, for example, 1.5 Vdd, 2.5 Vdd or 3.5 Vdd.